St. Anselm of Canterbury (1033–1109 CE) was a Christian philosopher. He was Archbishop of Canterbury from the age of 60 until his death.

The Ontological Argument

Anselm is perhaps most famous for developing the ontological argument for the existence of God.

1. God is that than which nothing greater can be conceived.
2. It is greater to exist than to not exist.
3. Therefore, God exists.

Anselm felt that the claim that God could exist in intellect (we know what God is supposed to be) but not in reality (where he would actually exist) was absurd. To demonstrate this, he walks through an argument with "the fool" who does not believe in God. The fool's argument is that God exists in intellect alone, and not in reality. If the fool also agrees that God is that than which nothing greater can be conceived, his argument falls apart:

1. God is that, than which nothing greater can be conceived.
2. God exists in intellect only, not in reality.
3. So, that than which nothing greater can be conceived exists only in intellect, and not in reality.
4. However, we can imagine a circumstance in which God exists, even if it is not the situation in our reality, we can imagine a world in which God does exist in reality.
5. It is greater for a thing to exist in reality and intellect than in intellect alone.
6. Therefore, we can conceive of a being greater than that than which nothing greater can be conceived.
7. But, this is absurd.
8. Therefore, the premise (2) God exists in intellect only, not in reality, must be false. God must exist in reality.

Ultimately, the argument says that than which nothing greater can be conceived must exist, because if it did not exist, we could always imagine something greater, which exists.

Anselm's claim says nothing about the nature of God — and certainly does not guarantee the existence of the Christian God. It only guarantees that if we can truly understand the definition of God, than God must exist.

Anselm's ontological argument has received criticism from a variety of sources, even within Christianity. The monk Gaunilo argues that the ontological argument proposed by Anselm could also be used to prove the existence of the perfect island, or the perfect anything. He argues that the jump from an intellectual concept to reality is not something within human capacity. Saint Thomas Aquinas argued against it on similar grounds, arguing that God's existence can not be known a priori.

Through his development of the ontological argument, Anselm is recognized as the founder of scholasticism, the medieval marriage of Christian theology and philosophy.